JPH0433005B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention relates to a projection lens for a television projector;
In particular, the present invention relates to a projection lens suitable for a three-tube television projector. (Prior art) A three-tube TV projector has B (blue) and G
(green) and R (red), each with fluorophores.
A color image is obtained by superimposing the CRT screen onto the screen using three lenses and projecting it in an enlarged manner. Therefore, the projection lens used for this purpose need not be an achromatic lens. However, the half angle of view is large at 20° to 25°, and the F number is also large.
It must have a large diameter of 1.0 to 1.4. It is known to manufacture this kind of lens from plastic, for example, JP-A-55-124114, JP-A-57-34515, JP-A-57-108818, JP-A-57
- Seen in No. 177115, etc. Furthermore, since it is composed of as few as three lenses and is made of plastic, it achieves significant cost and weight reductions compared to conventional glass lenses. On the other hand, television projectors are required to have large screens and be small in size. For this reason,
The optical system of a television projector consists of one or more flat reflecting mirrors placed between the projection lens and the screen, bending the optical path to make the device smaller, and to further miniaturize the device. It has also been proposed to use an in-mirror lens with a built-in mirror as a projection lens. Regarding the above-mentioned known lens, a flat reflecting mirror is inserted between the screen-side lens and the middle lens among the three lenses, and the lens has a large aperture.
Some cameras can be used with a wide angle of view, but
A flat reflector is inserted between the CRT screen side lens and the middle lens, or a flat reflector is inserted both between the screen side lens and the middle lens, and between the middle lens and the CRT screen side lens. In the form of,
I can't find anything that can be used with a large aperture and wide angle of view. (Objective of the Invention) The present invention provides a three-element lens with a large axial distance between the constituent lenses despite its large aperture and wide angle of view, making it possible to insert a reflecting mirror while reducing aberrations. The objective is to obtain a projection lens that is well-corrected. (Structure of the Invention) As shown in FIG.
The lens consists of a second lens that is a biconvex lens and a third lens that is negative, and at least one refractive surface of each lens is an aspherical surface, and −0.1<f/f ₁ <0.1 ...(1) 0.45< r ₃ ／｜r ₄ ｜＜0.9 …(2) −0.7f＜r ₅ −0.35f …(3) where f: Combined focal length of combined system f ₁ : Focal length of first lens r ₃ : Second lens Radius of curvature of the apex of the surface on the screen side of the second lens r ₄ : Radius of curvature of the apex of the surface of the second lens on the side opposite to the screen r ₅ : Radius of curvature of the vertex of the surface of the screen of the third lens Constructed as a lens system that satisfies the following conditions. be done. Condition (1) is a condition that defines the distribution of refractive power to the first lens. When the positive refractive power becomes strong beyond the upper limit, the distance between the second lens and the third lens must be shortened in order to lengthen the back focus. For this reason, if a plane reflecting mirror is inserted between the second lens and the third lens, a large aperture and a large angle of view cannot be achieved, and the lens cannot be used as a projection lens for a television projector. On the other hand, if the lower limit is exceeded and the negative refractive power becomes strong, the aperture and center thickness of the second lens will become significantly large. Condition (2) is a condition that defines the shape of the second lens. Since the refractive power of the first lens is weak and the angle of incidence of the axial light beam of the third lens is small, the refractive power of the entire system is almost entirely borne by the second lens. Therefore, if the shape of the second lens is not appropriate, even if an aspherical surface is introduced, off-axis aberrations cannot be corrected. Exceeding the upper limit is advantageous for correcting coma aberration, but astigmatism increases. Conversely, if the lower limit is exceeded, inward coma aberration occurs. Condition (3) defines the curvature of the screen-side surface of the third lens, and is a condition for correcting field curvature. Since the incident height of the axial light beam incident on this surface is small, it is possible to provide a large curvature without affecting the refractive power and spherical aberration of the entire system. When the curvature becomes strong beyond the upper limit, the curvature of field becomes excessive. Conversely, if the curvature becomes weak, the field curvature becomes under. Like a projection lens for a TV projector,
In order to obtain the desired performance with a lens having a large diameter and a wide angle of view, it is desirable to satisfy each of the above conditions and to configure at least one refractive surface of each lens to be an aspherical surface. Making the first lens aspherical corrects spherical aberration, making the second lens aspherical corrects spherical aberration and coma, and making the third lens aspherical corrects field curvature and aberration. Point aberration can be corrected. Examples of this invention will be shown below. In the table, r _i is the axial radius of curvature of each lens surface, d _i is the distance between each lens surface, n _i is the refractive index of each lens at a wavelength of 508 nm, ν _i is the Atsube number of each lens for the d-line,
K and A _i are the conic constant and the aspherical coefficient, respectively, when the aspherical surface is expressed by the following equations. However, the coordinate origin is the apex of the aspheric surface, the optical axis direction is X, the y axis is perpendicular to this, C is the vertex curvature P _i is the power of the aspheric surface, and φ=√ ² + ² . In addition, in the table, the face plate of the CRT tube,
Values related to G are also shown.

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[Table] (Effects of the Invention) As is clear from the examples and drawings, the lens of the present invention has a large axial air gap between each lens, and although a reflector etc. can be easily inserted, the lens of the present invention As seen in the aberration diagrams in Figures 2 to 4,
Aberrations are corrected for large apertures and wide angles of view, and it has sufficient performance as a projection lens for television projectors.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a first embodiment of the lens system of the present invention, and FIGS. 2, 3, and 4 are aberration diagrams of the first, second, and third embodiments, respectively. .

Claims (1)

[Claims] 1. A first lens with weak refractive power from the screen side,
Consisting of a second lens that is a biconvex lens and a negative third lens, at least one refractive surface of each lens is an aspherical surface, and −0.1<f/f ₁ <0.1 0.45<r ₃ /|r ₄ ｜<0.9 −0.7f<r ₅ <−0.35f where f: Combined focal length f ₁ : Focal length of _one lens r ₃ : Vertex curvature radius of the screen side surface of the second lens r ₄ : Second lens A projection lens for a television projector, characterized in that the radius of curvature of the vertex of the surface of the lens on the side opposite to the screen r ₅ satisfies the condition of the radius of curvature of the vertex of the surface of the third lens on the screen side.